Continuous miners of the milling or drum-type are well known to those skilled in the art. Each such mining machine or apparatus s incorporates two or more cutting drums or heads that are rotated parallel to the mineral face. Each of the cutting drums includes a series of picks that serve to cut aggregate material from the mineral seam such as a coal seam. Mining machines of this type are utilized in both continuous mining systems and highwall mining systems such as described in, for example, U.S. Pat. Nos. 5,112,111; 5,261,729 and 5,364,171 all to Addington et al.
Drum type continuous miners generally incorporate a gearcase of conventional design. As shown in FIGS. 1 and 1a, the conventional or prior art gearcase design includes a bevel gear set including an input pinion A and output bevel gear B. The output bevel gear B drives a hollow sun gear C which meshes with planetary gear set D carried by rotating planetary carrier E. The planetary gears D also mesh with a fixed ring gear F. It is the rotating planetary carrier E that drives the end cutting drum (not shown). A main drive shaft G connected to the planetary gear set carrier E drives the middle cutting drum (not shown).
In an alternative prior art embodiment shown in FIG. 1b, two matching mirror-image gearcases substantially corresponding in design to the gearcase shown and described in FIGS. 1 and 1a are interconnected by the main drive or cross shaft G.
It should be appreciated that the main drive shaft G in either of the above-described prior art embodiments must be of sufficient size and strength to transmit the horsepower from the drive motor (e.g. 125-350 horsepower) through the rotating planetary carrier E to the middle cutting drum. Accordingly, a solid, high-strength steel shaft of between 9-15 centimeters in diameter is typically employed. As shown in FIG. 1, the sun gear C must be of sufficient diameter to accommodate a centerbore of sufficient size to provide clearance for the main drive shaft G.
In all gearcase designs of this type, it is the size or overall diameter of the planetary gear set which dictates or limits the overall size of the gearcase. In this conventional design, the planetary gear set must orbit the sun gear C which must be sized of sufficient diameter to accommodate passage of the main drive shaft G. Since the main drive shaft G must be of sufficient diameter to transmit horsepower to the center cutting drum without failure, any possible reduction in the outer diameter of the planetary gear set D and therefore the gearcase is strictly limited. This in turn means that any possible reduction in the diameter of the cutting drums which fit concentrically over the gearcases is also limited. This in turn means that the mining apparatus is only capable of efficiently mining relatively thick seams: that is, seams having a thickness at least equal to or greater than the outer diameter of the cutting drums. The cost of cleaning the won coal becomes prohibitively expensive if an attempt is made to mine thinner seams.
As more and more of the world's coal reserves are contained in relatively thin seams, a need is therefore identified for an improved gearcase design that allows the outer diameter of the gearcase and its component parts to be reduced. This in turn allows the working diameter of the cutting drums to be reduced so as to allow efficient and cost effective operation in relatively thinner seams.